When an airplane pilot, or other user of a safety harness device, is to be ejected from an airplane in an emergency situation, it is important that the user be in an upright position in his seat in order to withstand the ejection forces involved without sustaining injury, and in order to avoid hitting the surrounding structure of the vehicle during the ejection process. It is also important that the retraction system be able to satisfactorily accommodate users of different weights during the various ambient conditions the user may encounter.
It is further desirable that the user be restrained when acceleration forces on the user relative to the vehicle exceed a certain level. At the same time, the user's normal movements should be unrestrained when acceleration forces are below this level. It is further desirable that the pull-back capability be operable regardless of the inertia forces affecting the inertia operated restraining capability. It is further highly desirable that the pull-back capabilities and the inertia operated restraint capabilities be combined into a single safety device.
U.S. Pat. No. 3,522,918--Wrighton discloses a safety harness device of the inertia type wherein a rewind spring in the device is connected to a fluid pressure actuated portion that is normally inoperative during ordinary operations of the safety harness. However, when actuated by the user, the fluid pressure actuated portion serves to pull upon the rewind spring to rapidly haul in the user, without injury, against the back of the seat prior to ejection, regardless of decelerating forces on the user at that time. That patent, which is incorporated herein by reference, utilizes the high pressure gases from an explosive device to provide the force in emergency situations for retracting a strap to haul back the user against his seat. The inertia responsive portion of the safety harness device disclosed in Wrighton is similar to that disclosed in U.S. Pat. No. 3,018,065--Cushman et al, which is incorporated herein by reference.
According to the Wrighton patent, a rewind spring means of the inertia responsive safety harness device, of the type disclosed in the Cushman, et al. patent, instead of being connected to the harness casing, is connected to a rotatable drum mounted in the ballistic portion of a safety harness device. That drum is normally fixed against turning during all normal operations of the device. However, should the pilot or user wish to eject from his vehicle, means are provided for rotating the drum so as to wind the rewind spring means thereon to cause turning of the reel spindle and effect rapid retraction of the strap, thereby moving the user's body rapidly but safely against the back of his seat preparatory to ejection.
More specifically yet, the Wrighton patent employs the high pressure gas to move a piston, which in turn drives a ball screw utilized to wind the retracting spring. To dampen the movement of the piston, in an attempt to provide some uniformity to the retracting force, back pressure is created by fluid on the backside of the piston. This fluid is forced out of a restricted fixed orifice to thereby create a dashpot effect for the piston movement.
A safety harness device of the type disclosed in the Wrighton patent has been very successful for many years and thousands of units have been utilized. However, more stringent specifications have now been established for the operation of such devices because of increased speeds and other capabilities in modern aircraft. For example, existing reels have been designed to haulback 98 percentile pilots undergoing 2 g's resistance acceleration in less than 300 milliseconds. In other words, a heavy pilot being pulled away from his seat by 2 g's acceleration must be hauled back in less than 300 milliseconds. At the other end of the performance scale, the reel must not haulback the 3 percentile (light) pilot (with no resistive acceleration) faster than 12 fps (feet per second) to prevent injury. It is desirable that a new system will be capable of hauling back the 98 percentile pilot undergoing 3.5 g's resistive acceleration in less than 200 milliseconds, while still limiting velocity of the pilot to 12 fps with the light (3 percentile) pilot.
This goal can be accomplished with the heavy pilot by simply increasing the retraction force, by increasing the actuating fluid pressure with a device of the type disclosed in the Wrighton patent. However, to be practical, the device must perform satisfactorily without injury to the user of a certain maximum weight, as well as a pilot of a certain minimum weight. In addition, the forces provided by a particular pressurized fluid are affected by ambient conditions. That is, with high ambient temperatures, the fluid pressure provided by a particular explosive device is greater than that provided by the same device with cold ambient temperatures.
As stated more succinctly, the desired device must be able to successfully haul back a pilot of a maximum weight on a cold day against high resistance acceleration forces and also be able to haul back the lightest pilot on a hot day with no resistance acceleration force, without injuring the lightweight pilot. While the device described in the above-referenced Wrighton patent satisfactorily accommodates various loads by the use of the above-mentioned dashpot feature, it cannot meet the newer, more stringent requirements needed in connection with modern military aircraft. If the system in the Wrighton patent is simply modified to provide the necessary force for the heaviest pilot on the cold day, the lightweight pilot on a hot day will likely be seriously injured because of the force with which such load will be hauled back into the upright position against the pilot's seat.
A problem of this sort has been addressed by a relatively complicated and expensive system wherein the velocity of the driving piston or actuator is sensed and the information is fed to a computer which controls a valve to control back pressure on the actuator. Primarily because of cost and size, such system has not been widely adopted.
Accordingly, a need exists for a simple, improved safety harness device which will meet the newer, more stringent requirements. Further, such improvement must be highly reliable and fit within the space limitations of the existing structure. More specifically yet, it is desirable that the improved system fit within the space envelope of the product currently made in accordance with the Wrighton patent.